A pressurized water nuclear reactor ("PWR") may contain water at about 625.degree. F. at a pressure on the order of 2250 psi.
Current PWR reactor designs allow for the use of movable in-core detectors, fixed in-core detectors, or a combination of both, to enable three-dimensional mapping of the thermal neutron or gamma ray flux distribution within the reactor core. To this end, a number of casings or thimbles lead into the core to allow for insertion and removal of the detector assemblies. These casings are generally in the form of much-elongated closed-end tubes that are intended to sealingly separate hot pressurized water within the reactor from a normally-dry chamber or compartment within the casing. The pressure within the thimble is normally at ambient atmospheric pressure. By inserting the detectors into these casings or thimbles, a utility may map the thermal neutron density within the core, and make certain adjustments to cause the fuel to be consumed evenly. Utilities typically map their reactor cores on a monthly basis. One example of an in-core gamma-compensated neutron detector is shown and described in U.S. patent application Ser. No. 07/769,140 filed Sep. 30, 1991, and assigned to the assignee of the present application, the aggregate disclosure of which is hereby incorporated by reference.
Some reactors are now equipped with automatic shut-off valves to prevent the escape of water in the event of a thimble leak. Heretofore, the leaking thimble was detected by water reaching a transfer mechanism, which alerted the operators to close a manual isolation valve communicating with the thimble. Another method to determine a breach of the sealed integrity of a thimble was in terms of the driveability of a movable in-core detector during mapping. If the thimble had developed a leak, the automatic isolation valve would close, preventing the operator from inserting a detector into the pressurized water-containing thimble. The thimble was labeled inoperative if the force needed to insert an in-core detector exceeds a predetermined value. Unfortunately, since core mapping is typically performed at discrete time intervals, there was no advance warning that a thimble had previously ruptured.
Moreover, stagnant water entering a thimble through a breach in its wall, will corrode various portions and components of such detectors. Upon information and belief, it has heretofore not been possible to determine such thimble failure electrically until such time as failure of the component has occurred. This may be some time after the thimble has developed a breach.
It would, therefore, be generally desirable to provide a means or mechanism for determining and monitoring the fluid-tight sealed integrity of a casing or thimble such that corrective action may be taken or scheduled promptly after a leak occurs, as opposed to first discovering the leak upon an attempt to insert or remove a detector or after a failure of the detector.
It would also be generally desirable to know that a thimble has ruptured promptly upon the occurrence of such event.